I've built a patch (base on r11840) to make redmine work in a subdirectory with WEBrick. I've used the ideas from this issue.

To use redmine in a sudirectory after applying this patch, you need to do two things:

Edit config/routes.rb to set the subpath at the top of the file.

Create a folder structure for the subpath in your public-directory, except for the last segment. So if your subpath is foo/bar/baz, create the folders foo and foo/bar. Then add a symlink into the deepest folder, pointing back to the public-directory and having the name of the last path segment as name. This symlink ensures that static files are served correctly.

This patch is far from perfect, especially the second part with the symlink needs improvement. If someone knows how to get around that, I'd be happy to add it to the patch.

Also, I did not test this with anything but WEBrick. So if you consider merging it, additional testing will be needed.